Water can pass relatively easy through the standard concrete block used for foundations. In an area with a high water table it is necessary to provide drainage around the underground foundation of a building if there is a subsurface room or cellar. It is usually not enough simply to waterproof the outside of the foundation walls. A tile drainage pipe is run along the foot of each buried foundation wall, and the foundation is externally waterproofed and backfilled with gravel. Thus water in the ground does not lie against the outer surface of the wall, but passes down through the gravel to the porous tile pipe.
This procedure has several drawbacks. During backfilling of the foundation, the waterproof layer--normally a thick coating of tar--is almost invariably damaged and pierced, so that leaks are formed. The gravel and drainage on the outside prevent such minor leaks from being troublesome except under very wet conditions, but the foundation walls still do leak at least some times. In addition the bituminous coating sometimes breaks down somewhat chemically, cracking and leaking. Finally in such arrangements the outer surface of the foundation walls, which are mainly of heat-conducting concrete, always is resting against the ground outside so that the foundation is quite cold and usually also somewhat damp if for no other reason than from condensation on the wall from inside the basement or cellar.
German patent Ser. No. 2,947,499 describes a polyethylene sheet lying generally in a plane and formed with bumps opening in one direction perpendicularly from the plane. Such an impact-resistant sheet is placed against the outside face of the foundation walls and holds the backfill out of contact with these walls. At the same time it forces water coming laterally at it to percolate down to the foot of the wall where, as described above, a porous tile pipe, normally bedded in gravel, is buried. Such an arrangement quickly silts up, that is becomes clogged with fine dirt, so that water can no longer move along it. Thus it stands against the held-back water for great periods of time, until it has all slowly percolated down, giving this water plenty of opportunity to leak through it. Furthermore, it transmits heat from the building out into the dirt, thereby leaving the foundation wall cold on the inside.
It is also known to place large bitumen-covered polystyrene-foam sheets against the outer wall surface. These are tarred in place or at least the joints are tarred to provide a waterproof insulating layer. Such panels are normally about 5 cm thick, 100 cm long, and 50 cm wide, so they are quite bulky, and therefore costly. They have not proven themselves to be physically or chemically stable in the long run, and are liable to silting up so they do not drain. This filling with fine dirt can be countered by applying a filter sheet, normally a porous but rigid textile web, to the outer face of the polystyrene sheets. Any gap in this arrangement makes, however a leak, and the panels are not particularly strong so they are easily damaged in transport or installation. Thus leaks are common.
Norwegian Pat. No. 107,188 of Bordewick shows an arrangement wherein an impervious sheet is formed with an array of inner bumps that hold it away from a wall. The other side of this foil, that is its side turned toward and contacting the ground, is smooth and is only interrupted by the irregularities formed by the bumps. The outer surface is otherwise smooth and uninterrupted, so that there is little drainage along this outer side.
U.S. Pat. No. 3,654,765 of Healy describes a foundation-drainage panel having an inflexible core element that is sandwiched between a water-pervious mesh and an impervious sheet. The entire arrangement is in fact quite rigid and is normally integrated with a rigid drain pipe. In addition the channels in this system are purely vertical, so that if stopped up they will fill and overflow above the element, forcing water behind the sheet against the wall. This type of arrangement can only be used against a flat building surface and cannot even be cut up conveniently to fit other than standard sizes. It can be bent about an axis parallel to its ridges, but not perpendicular thereto. Such deformation perpendicular to the ridges creases them and limits or eliminates flow along them. In addition in this arrangement the filter mesh overlies the ridges so that about half of its surface area is useless, as it lies directly against the crests of the ridges or bumps.